There is an increasing need to determine the presence of minute quantities of organic compounds in aqueous solutions and to quantify their concentrations. Concentrations of interest range from about 10.sup.-4 -10.sup.-12 molar and even lower. The determinations are most significant in areas of detecting the presence of drugs of abuse in physiological media, metering of therapeutic doses of drugs, and diagnosis of disease for which the presence, absence or amount of a particular organic compound is relevent to the diagnosis. Non-physiological areas of interest include investigations of water contamination, quality control and the like. Despite major advances in the immunoassay field, the precise determination of low concentrations of organic compounds continues to present problems, and many methods are very difficult to carry out in a routine manner with laboratory technicians.